GB1572767A

GB1572767A - Impeller for an axial flow fan

Info

Publication number: GB1572767A
Application number: GB3353/77A
Authority: GB
Original assignee: Torin Corp
Current assignee: Torin Corp
Priority date: 1976-01-28
Filing date: 1977-01-27
Publication date: 1980-08-06
Also published as: FR2339740B1; DE2703568A1; IT1082388B; US4063852A; FR2339740A1; DE2703568C2

Description

PATENT SPECIFICATION

Application No 3353/77 ( 22) Filed 27 Jan 1977 Convention Application No 653399 ( 32) Filed 28 Jan.

United States of America (US) Complete Specification Published 6 Aug 1980

INT CL 3 F 04 D 29/38 ( 52) Index at Acceptance F 1 V 104 CS ( 11) 1 572 767 ( 19) 1976 in &: 2 N ( 54) AN IMPELLER FOR AN AXIAL FLOW FAN ( 71) We, TORIN CORPORATION, Kennedy Drive, Torrington, Connecticut 06790, United States of America, a Corporation organized under the laws of the State of Connecticut, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:-

A wide variety of blade shapes have been designed for use in low pressure applications of axial flow fans over the years Blade parameters such as camber, pitch, and chord have of course been varied in arriving at desired impeller performance characteristics While a "cut and try" design technique has probably been most commonly employed more sophisticated design methods such as a "Free Vortex" design technique have also been used The resulting blades and impellers have been generally satisfactory but one or more problems of excessive size, noise generation, vibration, etc is usually encountered in operation.

It is the general object of the present invention to provide an optimum impeller blade design which represents a judicious compromise of design objectives such as minimum noise generation, small size and material economy.

According to the present invention there is provided an impeller for an axial flow fan for low pressure ratio application in the range 1 03 and below; the impeller comprising a hub adapted for rotation about an axis and carrying a plurality of similar circumferentially disposed air moving blades, each of said blades having a root portion attached to the hub and a radially outwardly disposed tip portion with smoothly curving side edges therebetween and wherein the chord measurement of each blade is substantially less at its root portion than at its tip portion, substantially all of the blade chord change occurring over the radially outermost 50 % of the blade span, and further wherein the camber of each blade decreases from the root portion to the tip portion, the blade camber decreasing gradually from the root for a first portion of each blade span and sharply for a second span portion disposed radially outwardly of the blade span mid point.

Further according to the present invention there is provided an axial flow fan comprising an impeller as described in the immediately preceding paragraph.

Variations of camber, pitch and chord are possible within the scope of the present invention and the preferred range of these variations will become clear in the description of one embodiment of the impeller with reference to the accompanying drawings, in which:Figure 1 is a front view of an axial flow air impeller constructed in accordance with the present invention.

Figure 2 is a rear view of the impeller of Figure 1.

Figure 3 is a top view of the impeller.

Figure 4 is a sectional view through a tip portion of an impeller blade taken generally as indicated at 4-4 in Figure 1.

Figure 5 is a sectional view through an intermediate portion of the blade taken generally as indicated at 5-5 in Figure 1.

Figure 6 is a sectional view through a root portion of the blade taken generally as indicated at 6-6 in Figure 1.

Figure 7 is a plot of normalized blade span fraction versus normalized change in camber.

An impeller for use with an axial flow fan and constructed in accordance with the present invention includes a hub and a plurality of similar circumferentially arranged air moving blades Both the hub and the blades may vary widely in construction and the impeller shown in the drawings is to be regarded as an illustrative example only The ( 21) ( 31) ( 33) ( 44) ( 51) C_ Z t_ pl tn 1,572,767 impeller shown is of molded thermoplastics construction but it will be obvious that materials of construction may also vary within the scope of the invention.

A hub 10 of the impeller shown in the drawings has a central opening 12 for mounting on an output shaft of, for example, an electric motor, and carries radially outwardly projecting air moving blades 14 Five air moving blades are shown and the blades are formed integrally with the hub at radially inwardly disposed or "root" portions 16 The blade configuration may also vary within the scope of the invention but generally within limits explained more fully hereinbelow.

As indicated above, blade configuration represents a judicious compromise of design objectives including minimization of noise generation, small size, and economy of material for given performance requirements.

The design method employed is relatively sophisticated involving computer calculation and assignment of given increments of work to the various "spanwise slices" or increments of the blade in order to meet overall blade performance requirements That is, the blades are designed with reference to blade "slices" or increments which extend across the blade and which are displaced one from the other by varying radial distances from blade root to blade tip or in a "spanwise" direction, blade span being measured from root to tip More specifically, the blades are so designed that a major portion of the required work is accomplished in intermediate portions of the blade, or throughout the "slices" or "increments" which are spaced from the blade end portions Blade root portions are unsuited to the assignment of a heavy work load as velocities in these regions are relatively low and, at the blade tip portions configurations which provide heavy work output also result in objectionable noise levels due to vortex interaction In contrast, intermediate blade portions are favored with substantial velocity and thus capable of a relatively heavy workload In a typical blade design in accordance with the present invention, the intermediate blade portion e g from 30 % to 80 % of blade span is designed to accomplish approximately % of the required overall work of the blade.

The treatment of the tip portions of the blades of the present invention is perhaps of greatest import in the analysis and design procedure As mentioned, the assignment of a heavy workload to these blade portions and the resulting blade tip configurations entail objectionable noise generation Particularly in the case of a shrouded impeller a complex vortex system exists in the region of the blade tip and results in a major portion of the high frequency noise generated by the blade.

Accordingly, the tip portions of the blades, approximately the outermost 20 % of blade span, may be designed to provide a modest amount of work but consideration of noise generation, size, and material economy are paramount in this region Blade camber is 70 sharply reduced beyond the blade midpoint and particularly toward the tip of the blade and the blade chord is preferably sharply increased in a similar region In this manner, high frequency noise generation is sharply 75 reduced, size and material conservation considerations are given due attention and yet the chord increase compensates at least in part for camber reduction and a modest but significant work output is achieved 80 As will be apparent, a substantial root to tip change in camber and chord occurs in the blade configuration of the invention In most instances blade pitch will also change (viewed from root to tip) and in the impeller 85 shown all three parameters change in the root to tip progression More particularly, blade camber decreases from root to tip for the blades 14, blade chord increases from root to tip, and blade pitch decreases, all 90 within preferred limits as set forth hereinbelow.

The change in blade camber is perhaps most important to the success of the present design and preferably should be within the 95 following limits, all values being given for mean blade camber Root camber C Ar should fall in the range 15 to 1 and tip camber C At in the range 020 to 040, with a maximum ratio of root to tip camber of 7 5 to 100 1, a minimum ratio of root to tip camber of 2.5 to 1 and a preferred ratio of 5:1 The specific values for the blade design shown are 1203 for the root camber C Ar, Figure 6, 023 for the tip camber C At, Figure 4, and a 105 ratio of 5 23 to 1 Figure 7 is a graph plotting normalized blade span fraction on the vertical axis versus normalized change in camber on the horizontal axis The normalized span fraction is given by the formula (Ts 110 Ti)( 8 To Ti) where:

To is the tip radius Ti is the root radius and T is the section radius.

The normalized changed in camber is 115 given by the formula ( O O)/Oi Oo) where:

Oo is the camber at the tip Oi is the root camber and Os is the section camber.

With particular reference to Fig 7, it is to 120 be observed that camber decreases gradually for a first portion of each blade extending from the root portion and that a sharp change in camber occurs for a second blade portion radially beyond the blade midpoint The said 125 second blade portion commences at approximately 70 % to 90 % of blade span and, more particularly, at approximately 80 % of blade span measured from root to tip In case of the blades 14 shown, approximately 35 % of the 130 3 1,572,767 3 overall camber change occurs in the final or outermost 20 % of blade span, Fig 7.

Further in accord with the invention, the increase in blade chord from root to tip is defined by a tip to root ratio which should not exceed 2 5 to 1 The lower limit of the tip to root ratio is 1 3 to 1 and the actual ratio for the blades 14 falls in the desired range at a value of 1 7 to 1, the blade tip chord C Ht measuring 2 6 inches ( 66 mm), root chord C Hr 1 5 inches ( 38 mm), and the intermediate chord C Hi Figure 5, measuring 2 2 inches ( 56 mm) The aforementioned second or radially outwardly disposed blade portion is also characterized by a sharp increase in chord and at least 80 % of the aggregate chord change occurs through the said blade portion As will be observed with the blades 14 substantially all of the blade chord change occurs over the outermost 50 % of blade span and, more particularly, over the outermost to 40 % of blade span.

Figures 1 and 2 of the drawings are elevational views and misleadingly give the impression of substantial blade chord change over the radially innermost 50 % of blade span.

This is clearly not the case, as will be appreciated from the above chord dimensions, and appears so in these Figures because of the decreasing pitch with increasing span Figures 1 and 2 should be compared with Figure 3 to give a more accurate visual impression of the preferred blade shape.

While the specific plan form of the blades at the outermost region of maximum chord may vary within the scope of the invention, it is preferred to provide a gradual arcuate edge as at the trailing edges 22 of the blades 14 Other edge configurations are acceptable, however, as at the leading edges 24 of the blades 14 where a relatively sharp or pointed configuration is provided The impeller shown is of the shrouded type and the sharp leading edges are determined by shroud configuration, the inlet side of the shroud being of a somewhat smaller diameter than the discharge side and the blades 14 conforming thereto That is, the blade span or radial dimension is slightly reduced at the blade leading edge, thus somewhat sharpening an otherwise gradual arcuate edge.

Blade pitch may also vary from root to tip as mentioned and the limits of such variation as presently contemplated include a root pitch angle ar between 300 and 700 and a tip pitch angle at between 100 and 300 Pitch ratios presently regarded as optimum limits within the scope of the invention include a maximum ratio of 7 to 1 from root to tip and a minimum ratio of 3 to 2 The root pitch angle ar for the blades 14 is approximately 460 (Figure 6), the tip pitch angle at approximately 27 and the intermediate pitch angle ai approximately 290 The actual ratio of root to tip pitch for the blades 14 is thus 1 7 to 1.

The results achieved with impeller blades constructed in accordance with the present invention include the minimization of size, noise generation and significant conserva 70 tion of material for given blade performance requirements With regard particularly to noise generation, a 5 to 6 decibel improvement on the USA Standards Institute or OSHA "A" Scale has been achieved with an 75 impeller having blades of the present construction in comparison tests with an impeller having conventional blades of substantially constant camber, chord and pitch characteristics An improvement in noise 80 characteristics of such magnitude is regarded as an outstanding advance in the fan industry.

Claims

WHAT WE CLAIM IS:-

1 An impeller for an axial flow fan for 85 low pressure ratio application in the range 1.03 and below; the impeller comprising a hub adapted for rotation about an axis and carrying a plurality of similar circumferentially disposed air moving blades, each of said 90 blades having a root portion attached to the hub and a radially outwardly disposed tip portion with smoothly curving side edges therebetween and wherein the chord measurement of each blade is substantially less at 95 its root portion than at its tip portion, substantially all of the blade chord change occurring over the radially outermost 50 % of the blade span, and further wherein the camber of each blade decreases from the root portion 100 to the tip portion, the blade camber decreasing gradually from the root for a first portion of each blade span and sharply for a second span portion disposed radially outwardly of the blade span mid point 105

2 An impeller as claimed in claim 1 wherein the camber at the root portion of each blade does not exceed 15 and the camber at the tip portion is at least 02.

3 An impeller as claimed in claim 1 or 110 claim 2 wherein the camber at the root portion of each blade is at least 1 and the camber at the tip portion does not exceed 04.

4 An impeller as claimed in any one of 115 claims 1 to 3 wherein the ratio of camber at the blade root portion to camber at the blade tip portion is no higher than 7

5 to 1.

An impeller as claimed in any one of the preceding claims wherein the ratio of 120 camber at the blade root portion to camber at the blade tip portion is no lower than 2 5 to 1.

6 An impeller as claimed in claim 4 or claim 5 wherein the ratio of blade root camber to tip camber is 5 to 1 125

7 An impeller as claimed in claim 2 or claim 3 wherein camber at the blade root portion is 12 and camber at the blade tip portion is 02.

8 An impeller as claimed in any one of 130 1,572,767 1,572,767 the preceding claims wherein the pitch at the root portion of each blade does not exceed 700 and the pitch at the tip portion is at least 100.

9 An impeller as claimed in any one of the preceding claims wherein the pitch at the root portion of each blade is at least 300 and the pitch at the tip portion does not exceed 300.

10 An impeller as claimed in any one of the preceding claims wherein the ratio of gitch at the blade root portion to pitch at the lade tip portion is no higher than 7 to 1.

11 An impeller as claimed in any one of the preceding claims wherein the ratio of pitch at the blade root portion to pitch at the blade tip portion is no lower than 3 to 2.

12 An impeller as claimed in any ofneof claims 8,10 and 11 wherein pitch at the blade root portion is 46 and pitch at the blade tip portion is 270.

13 An impeller as claimed in any one of the preceding claims wherein the ratio of blade root pitch to tip pitch is 1 7 to 1.

14 An impeller as claimed in any one of the preceding claims wherein the ratio of chord at the blade tip portion to chord at the blade root portion is no higher than 2 5 to 1.

An impeller as claimed in any one of the preceding claims wherein the ratio of chord at the blade tip portion to chord at the blade root portion is no lower than 1 3 to 1.

16 An impeller as claimed in claim 14 or claim 15 wherein the ratio of blade tip chord to root chord is 1 7 to 1.

17 An impeller as claimed in claim 16 wherein the chord measurement at the blade root portion is 1 5 inches ( 38 mm) and the chord measurement at the blade tip portion is 2 6 inches ( 66 mm).

18 An impeller as claimed in any one of the preceding claims wherein the second blade span portion commences in the range of 70 % to 90 % of blade span measured from root to tip.

19 An impeller as claimed in claim 18 wherein the second blade span portion of each blade commences at 80 % blade span measured from root to tip, and wherein 35 % of the camber decrease occurs in the second blade span portion.

An impeller as claimed in any one of the preceding claims wherein a sharp increase occurs in blade chord over the radially outermost 50 % of the blade span.

21 An impeller as claimed in claim 20 wherein at least 80 % of the aggregate chord change occurs in the second blade portion.

22 An impeller for an axial flow fan, the impeller being substantially as herein described with reference to the accompanying drawings.

23 An axial flow fan comprising an impeller as claimed in any one of the preceding claims.

URQUHART-DYKES & LORD, Chartered Patent Agents, 11th Floor, St Martin's House, Tottenhain Court Road, London W 1 P OJN -and3rd Floor, Essex House, 27, Temple Street, Birmingham, B 2 5 DD Agents for the Applicants Printed for Her Majesty's Stationery Office.

by Croydon Printing Company Limited, Croydon, Surrey, 1980.

Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A i AY, from which copies may be obtained.